I went shooting this afternoon with some folks, including Lawrence and Mike the Musicologist. Then we went to dinner. And at dinner, I came up with an idea. Even better, my idea has almost nothing to do with my previous post. (It does use some of the same technology.)
Those of my readers who are People of the Gun probably know what a ballistic chronograph is. For those who aren’t: briefly, a ballistic chronograph measures how fast a bullet is going. You place a stand that has two evenly spaced “screens” in front of your gun, and then fire a bullet through the screens. As the bullet passes through each screen, the screen detects the bullet’s passage. The screens are a known distance apart, so by measuring the time difference between the bullet’s passage between the screens, you can determine how fast the bullet is going. Typically, the screens are connected by a cable to a “head unit” that displays the velocity of the last shot, as well as keeping records for all shots in a session (including averages and standard deviations).
Ammunition catalogs will typically give you a muzzle velocity, but that assumes a certain barrel length, certain atmospheric conditions, and other factors. If you really want to know how fast a particular load from your gun is – or if you load your own ammunition, which wouldn’t be in a catalog – you want a ballistic chronograph. (Knowing velocities is also important in determining trajectories; for example, how much a bullet of a certain shape that starts out at a certain velocity will drop at 100 yards, 200 yards, 300 yards, and so on.)
That’s kind of a simplified introduction. Here’s my idea: why not put Bluetooth into the base that has the screens on it? And then, make your “head unit”, the part that displays the velocity and calculates averages, standard deviation, et al, an iPhone/Android app? You pair the screen base with your phone using the app; the phone has all the smarts and does all the calculations. You probably don’t need anything more complex in the base than the equivalent of a Bluetooth headset; just something to send the elapsed time over to the phone.
I see two good things about this: first, you’re saving some money on hardware because you don’t need a “head unit”, just the screens and an app. Second. when you want to upgrade the chronograph with “additional features”, all you really need to do is upgrade the phone app; the piece with the screens can be really dumb, since it just measures elapsed time between the bullet crossing the screens. All the real effort can be handled by the app that communicates with the screens.
Standard Bluetooth (like your wireless headset uses) has a range of about 30 feet, or 10 yards. My understanding is that most people put their chronograph screens somewhere around 5 yards from the muzzle, so that’s maybe 15 or 20 feet. We can figure that there won’t be any obstructions between the phone (which is probably sitting on the shooting bench) and the screen unit, so Bluetooth ought to work for this. If we’re really worried, we could make the link Bluetooth Class 1 instead of Class 2, which would give us about 100 yards of range between the screen unit and the phone, but I think that’s probably overkill.
The only possible drawback I can see is power: the screens are going to need their own power source, but you could easily put a fairly large rechargeable battery into the base of the unit. Enabling Bluetooth on the phone and using the app is going to suck up some battery power, but I’m thinking no more so than a Bluetooth headset. I’d be surprised if you couldn’t get at least two to three hours of chronographing with your iPhone before it needed recharging (depending on how much of a charge you started with). If your sessions at the range testing hand loads run all day, an external battery pack for your phone solves that problem.
A quick Google search turns up nothing like this idea, and I don’t see any reason why it can’t work. If you do, please post in comments. If you like this idea and have the skills to build it, you’re welcome to take the ball and run with it; I’d put up a Kickstarter for it, except I have no EE skills that would enable me to build this device, and I don’t know anyone who does have those skills.
I’m not going to pretend I have solid evidence, here, but considering that you’re talking about an application where milliseconds are critical, the engineer in my brain started screaming about latency when you started talking about putting all the intelligence in an app on a phone or tablet, and doing all the communication over Bluetooth.
Were I designing this, I’d use a dedicated (and wired, not wireless) embedded system in the base for the measurement and computation, and just send processed data (sure, use Bluetooth if you feel like it) to the head-end app. Your overall end-to-end latency won’t change meaningfully, but it’ll all be moved into the part of the process where milliseconds don’t matter anymore.
Lelnet:
I agree with you with respect to the latency issue, and Lord knows you have more experience than I do with this. But my thought was that the base unit would just measure the elapsed time between transits of the screens and pass that data off to the phone or tablet app, which would take the time value and do the calculations.
There’s probably no reason why you can’t have the base do the actual velocity calculation (based on elapsed time) as well and hand that off to the phone or tablet; it’d require a little more processing power in the base, but maybe not too much more? I think you still get the big wins of having much of your intelligence in the phone or tablet.
One of those big wins that I didn’t go into (because it didn’t occur to me until after that post) is the ease of exporting data. It would be just A Small Matter Of Programming to have the app create a .CSV of shot data and send it by email/Bluetooth file transfer/whatever to a computer. My impression is that exporting data from current ballistic chronographs can be done (at least with the higher end ones) but isn’t easy.
And there’s all sorts of data you can add to the .CSV, too: GPS coordinates, including elevation. Ambient temperature. Wind speed using a Kestrel, like the Shooter app.
You’re a lot smarter than I am, though, so if you see problems, I will listen to you with great respect in this matter.
Ah, OK…you were already thinking of doing the velocity comp in the base. Which completely eliminates my main objection. As long as the base-to-head traffic amounts to messages of the approximate sense of “a shot just came through now-ish, and the bullet was going X m/s”, there’s no reason not to build the analytical portion of the intelligence into the phone/tablet app.
Indeed, I seem to remember a post in the recent past on “Ambulance Driver” reviewing a similar system that used a laptop app (and I think fed it by USB…pardon me for being too lazy to go back for the details, but since he’s on your blogroll I’m just going to assume you have some memory of the review I’m talking about). No reason it couldn’t be a phone and bluetooth…although personally, I’d actually prefer a laptop to a phone, for anything I’m going to want to be doing post-processing on.
(Dammit, I’m only 38! I am not SUPPOSED to be an oldfogey yet! It’s not like I’m asking for a System 360 mainframe and a room full of 3270 terminals, here! Just something with a proper keyboard and an OS that understands standard I/O. 🙂 )
But seriously…even when I thought you were talking about sending 2 bluetooth signals to the phone and asking the app to compare their arrival times to compute the speed, it sounded like the seed of an awesome idea that just needed some tweaking to bring it into compliance with reality. And now that I know that you weren’t talking about that…well, I’d contribute to that Kickstarter. 🙂